The present invention relates to a method for determining the amount of charge which can be drawn from a storage battery by measuring battery current values and battery voltage values at least two points in time before or during a rise phase and during or after a decay phase of a charging or discharging operation. The invention also relates to a monitoring device for a storage battery with measuring means for measuring battery voltage values and battery current values and with evaluation means.
Numerous methods are known for determining and predicting the state of a storage battery by simple means during the operation of the storage battery.
For example, DE 199 50 424 A1 describes a method for determining the starting capability of a starter battery of a motor vehicle, in which the battery current and the battery voltage are determined shortly before and during loading of the starter battery by starting of the internal combustion engine. The internal resistance and the amount of charge drawn are calculated from the respective pairs of values of the battery current and the battery voltage. A measure of the availability of the starter battery during the starting operation is derived from the rate of rise of the internal resistance against the amount of charge drawn.
DE 37 12 629 C2 describes a measuring device for determining the remaining lifetime of a motor vehicle battery, which registers the battery voltage and the associated load current value before and after starting for the first time with the storage battery in the fully charged state. Moreover, the temperature-compensated internal resistance is determined and stored in a memory. This initial temperature-compensated internal resistance is compared with internal resistance values which are determined for later starting operations of the internal combustion engine by the motor vehicle battery. The indication of the expected lifetime of the storage battery takes place thereafter in dependence on given stored threshold values.
It is known from WO 99/17128 to compare during the starting operation the voltage values of neighboring voltage minima, which occur on account of the compression and decompression of the engine pistons of an internal combustion engine started with the aid of the storage battery. The difference between these values serves as a measure for an indication of the state of the battery.
DE 39 01 680 C2 describes a method for monitoring the cold starting capability of a starter battery of an internal combustion engine, in which the variation over time of the voltage drop across the connection terminals of the starter for the internal combustion engine during the starting operation is observed and evaluated.
DE 198 47 648 A1 discloses a method for determining the state of charge and the peak current loadability of storage batteries, in which the no-load voltage is determined, the true open-circuit voltages are determined, with the variation over time being taken into account in particular, the converted amount of current is determined and the acid capacity is determined. An indication as to the starting capability of the storage battery is derived from this.
WO 00/04620 describes a system and a method for observing the state of a storage battery which is installed in a vehicle. An internal battery resistance and a polarization resistance are calculated from the battery voltage and battery current values determined during the starting of the engine. In addition, a value for the state of charge SOC is determined. On the basis of these values and their rates of rise, the battery state is analyzed and an indication given as to whether the battery is still capable of starting the engine.
Although the methods and devices described above serve for determining the suitability of a storage battery with regard to the capability of starting an engine or ascertaining the state of charge of a new battery, they do not allow an indication to be given as to the amount of charge which can be drawn from the storage battery, in particular for small currents.
One issue with storage batteries is, however, that the amount of charge Q which can be drawn when a storage battery is fully charged can decrease due to various causes. Consequently, the same amount of charge as in the new state can no longer be drawn from a used storage battery. These causes may be, for example, in the case of lead-acid storage batteries, the loss of active mass due to sludge deposition, sulfation or the like.
If, for example, the state of charge of a storage battery is determined by measuring the open-circuit voltage, as is possible for example in the case of a lead-acid storage battery, this value does not allow any indication to be given as to which amount of charge can still be drawn from the storage battery if the latter is no longer in the new state. The reason for this is that, although the state of charge is in this case a measure of the amount of charge which can be drawn from the acid, the amount of charge which still can be drawn from the active mass only correlates with the amount of charge in the acid in the new state.
With a definition of the state of charge SOC as a quotient of the difference between the nominal amount of charge and the amount of charge drawn with respect to the nominal amount of charge   SOC  =            (                        nominal          ⁢                                          ⁢          amount          ⁢                                          ⁢          of          ⁢                                          ⁢          charge                -                  amount          ⁢                                          ⁢          of          ⁢                                          ⁢          charge          ⁢                                          ⁢          drawn                    )                      nominal        ⁢                                  ⁢        amount        ⁢                                  ⁢        of        ⁢                                  ⁢        charge            ,      the state of charge SOC likewise does not provide a correct indication as to the amount of charge Q which can be drawn from the storage battery.
According to these definitions, the state of charge consequently does not provide any indication as to the amount of charge Q which can be drawn in the full state of charge. Since in many technical applications the state of charge SOC can only be determined by these methods, there is the need for a method which determines the amount of charge Q which can actually be drawn. There is also a need for a device (e.g., a monitoring device) for use with a storage battery that is capable of measuring battery voltage and current values.